The knee joint includes three bones, namely the femur, tibia and patella. The distal end of the femur lies adjacent to the proximal end of the tibia while the patella lies adjacent the anterior portion of the femur. The joint elements that engage one another are preferably covered by articular cartilage. Specifically, the distal end of the femur and the proximal end of the tibia are covered by articular cartilage, as is the posterior surface of the patella.
The articular cartilage of the knee joint may become damaged due to degeneration and/or wear, which may lead to bone-to-bone contact during articulation of the joint. This may result in significant pain and potential damage to the bone surfaces. A knee replacement procedure may be required in cases where damage to the articular cartilage is significant. Depending on the extent of the damage, the procedure may include at least partial replacement of one or bone of the knee joint. For instance, in a total knee replacement each of the bones of the knee joint is at least partially covered by implants. In other instances, a knee arthroplasty procedure may be limited to portions of one of the joints. For instance, a patellofemoral procedure is limited to at least partial replacement of the engagement surfaces between the femur and the patella.
The medial and lateral condylopatellar notches represent a border between the patellofemoral and tibiofemoral articulations of the distal femur. The condylopatellar notch is a concave depression formed between the patellofemoral compartment and the medial and lateral compartments respectively. The concave depression is formed at least in part due to femoral contact with the tibial meniscus when the leg is extended. The geometry of the concave depression is in contrast to the convex geometries of the adjacent compartments. The sulcus of the condylopatellar notch forms a spline that represents a transition region or functional boarder between the compartments of the knee. This spline can be simplified to a best-fit line which substantially describes the location and trajectory of the border.
Conventional patellofemoral implants typically cover the femoral trochlea only and generally require that a lateral tibiofemoral component, for example, extend past the condylopatellar notch into the patellofemoral compartment. In the case where no lateral tibiofemoral implant is implanted, a surgeon must carefully plan the transition between the patellofemoral implant and the native cartilage. In such cases, it is expected that the patella will frequently transition from the patellofemoral component to the native condylar cartilage during active and deep flexion.